The present invention relates to an inverter unit that drives a three-phase motor.
Japanese Unexamined Patent Application Publication No. 2005-192358 discloses a two-phase modulation control inverter unit as an inverter unit that drives a three-phase motor. The two-phase modulation control inverter unit drives a three-phase motor by estimating angular position (or electrical angle of the rotor) of the three-phase motor based on the electric currents flowing through the U phase, V phase and W phase of the three-phase motor computed from voltage applied to a shunt resistor, by switching ON and OFF the switching devices that send alternating currents to any two phases of the three phases based on the estimated angular position and also by keeping the switching devices that send an alternating current to the third phase in either ON or OFF state.
This two-phase modulation control inverter unit can reduce the switching loss as compared to a three-phase modulation control inverter unit wherein the switching devices of all three phases are switched ON and OFF.
In the case of a two-phase modulation control inverter unit for a three-phase motor that requires a low voltage, for example, when the two-phase modulation control inverter unit drives the three-phase motor at a low speed, however, the time during which the switching devices of the two phases of the three phases are ON is reduced and hence the time during which electric current flows through the shunt resistor is also reduced. In a read cycle in which the control circuit such as CPU reads voltage applied to the shunt resistor, if the pulse width of a control signal that switches ON the switching devices is smaller than the pulse width corresponding to the time that is long enough for the control circuit to read voltage applied to the shunt resistor, the control circuit fails to read the voltage, which may deteriorate the controllability of the three-phase motor.
When the pulse width of the control signal that switches ON the switching devices is smaller than the pulse width corresponding to the time that is long enough for the control circuit to read voltage applied to the shunt resistor in the above-described read cycle, the pulse width of the control signal that switches ON the switching devices needs to be increased in the next voltage read cycle. Referring to FIG. 8, when the read cycle of the voltage applied to the shunt resistor is three times as large as the signal cycles of the carrier signal used for pulse width modulation (PWM) control and also when the voltage is read at the time of outputting of a pulse of the control signal for the U phase computed at the third signal cycle of the carrier signal, the pulse width computed at the third signal cycle of the carrier signal needs to be made larger than those of the other two signal cycles.
When the pulse width of the control signal that switches ON the switching devices is increased in the next voltage read cycle, however, the switching devices are switched ON for an unnecessarily long time, which may deteriorate the controllability of the three-phase motor.
The present invention is directed to providing an inverter unit that drives a three-phase motor by switching ON and OFF the switching devices of two phases of three phases and also by keeping ON or OFF the switching devices of the third phase, thereby preventing the controllability of the three-phase motor from deteriorating.